Ordered Dewetting and Self-Organization of Microdroplets on Surfaces

Abstract

1Introduction 1. 1 Dewetting Dewetting is one of the processes that can occur at a solid-liquid or liquid-liquid interface. 1) Generally, dewetting describes the rupture of a thin liquid film on the substrate (either a liquid itself, or a solid) and the formation of droplets. The opposite process spreading of a liquid on a substrateis called wetting. The factor determining the wetting and dewet-ting is the so-called spreading coefficient S Sδ Sδ Fδ SF (1) where δ S and δ F are the surface free energies of the substrate and film, respectively, and δ SF is the interfa-cial energy between film and substrate. 2) When S 0, the surface is considered wettable, and if S 0, dewetting occurs. Wetting and dewetting are important processes for many applications, including adhesion , lubrication , painting, printing, and protective coating. For most applications, dewetting is an unwanted process, because it destroys the applied thin film. In most dewetting studies a thin polymer film is spun onto a substrate. Even in the case of S 0 the film does not dewet immediately if it is in a metas-table state, e. g. if the temperature is below the glass transition temperature of the polymer. Annealing such a metastable film above its glass transition temperature increases the mobility of the polymer chains and dewetting takes place. When starting from a continuous film, an irregular pattern of droplets is formed. The droplet size and droplet spacing may vary over several orders of magnitude , since the dewetting starts from randomly formed holes in the film. 38) There is no spatial correlation between the dry patches that develop. These dry patches grow and the material is accumulated in the rim surrounding the growing hole. In the case where the initially homogeneous film is thin (in the range of 100 nm), a polygon network of connected strings of material is formed, 9) like a Voronoi pattern of polygons. 10) These strings then can break up into droplets, a process which is known as the Rayleigh-Taylor instability. 11) At other film thicknesses, other complicated patterns of droplets on the substrate can be observed, 12) which stem from a fingering instability of the growing rim around the dry patch. 1. 2 Dewetting on patterned surfaces For many applications in fields ranging from biology to photonics and electronics, surface patterning is essential. The production of high-performance devices with micrometer features is usually based on liThe formation of ordered dewetted patterns of organic materials on substrates is described. The driving force for dewet-ting is the interfacial tension between substrate and organic solution. The observed regular order of the formed microdrop-lets, or "microdomes" can be explained by so-called dissipative structures, a concept developed by Ilya Prigogine. The apparent violation of the second law of thermodynamics (which should prohibit formation of ordered structures from iso-tropic solution) can be explained by the overall increase of the entropy by the evaporating solvent. The role of the substrate in the formation of ordered dewetted microstructures will be discussed briefly. In the second part of this paper examples for the application of dewetted microstructures in the field of photonics will be given. Special emphasis will be made not only for polymeric but also for low molar mass compounds.